1. Field of the Invention
This invention is directed generally to scanning systems which must precisely locate image picture elements (pixels) throughout a flat format during writing or reading operations. More particularly it relates to the positioning of image pixels in both X and Y directions on a format so as to correct height errors as well as spacing errors with a high order of precision compatible with extremely small spot size.
2. Description of the Prior Art
In high speed scanning systems such as precision plotters, printers, and the like, a number of problems are encountered in exactly locating image pixels throughout an object field format, particularly a flat format. The scanning beam, which is moved transversely across an object plane (here called the X direction) is equally of non-constant velocity and must traverse non-uniform path lengths. Errors can thus occur in the X direction due to optical distortion, scanner speed, projection of the scan onto a flat format, or simply because of vibration of the equipment. Discussion of such problems can be found in an article entitled "Generation of Precision Pixel Clock in Laser Printers and Scanners" by Gerald Toyen, published in SPIE, Volume 84, Laser Scanning Components and Techniques (1976), pp. 138-145.
In the aforementioned article by Toyen, it is proposed to split a laser beam prior to modulation and direct an auxiliary, unmodulated, beam in parallel with a main beam so as to be reflected off a polygon and through an optical grating which functions as a positional reference. Thus by synchronizing data to a pixel clock derived from the reference, one precisely locates pixels in the X-direction whether reading or writing, although the systems may otherwise be substantially different.
The aforementioned auxiliary beam clock does not, however, correct or eliminate height (Y-direction) errors on the format. These errors can occur due to scan inaccuracies such as non-parallelism of the faces of a scanning polygon, vibration, table drive errors, and the like. Since such errors affect the linearity as well as placement of a line of pixels, they determine the vertical resolution of the system, and should be minimized to a degree consistent with the system application. Mechanical and optical systems can only be made highly precise at inordinate expense, while at the same time modern scanning systems may use focused beams of the order of 15 .mu.m in diameter and pixels each 0.0005". Systems of the prior art do not provide a basis for economically achieving such resolutions in both X and Y directions.
U.S. Pat. Nos. 4,306,242, issued Dec. 15, 1981 to E. A. Jeffery; 4,310,757, issued Jan. 12, 1982 to F. T. Check, Jr. et al; 4,311,384, issued Jan. 19, 1982 to W. H. Keene; and 4,311,385, issued Jan. 19, 1982 also to W. H. Keene; disclose additional examples of laser recording systems and the like, which employ an auxiliary or reference beam for clocking purposes. Another example of laser scanning apparatus having beam position means can be found in U.S. Pat. No. 4,279,472, issued July 21, 1981, to G. S. B. Street, while other examples of clocking arrangements for laser beam recording systems, and the like, can be found in U.S. Pat. Nos. 4,212,018, issued July 8, 1980 to M. Ohnishi et al; 3,389,403, issued June 18, 1968 to J. M. Cottingham; 4,285,012, issued Aug. 18, 1981 to Y. Ohara et al; 4,293,202, issued Oct. 6, 1981 to M. Ohnishi et al; and 4,307,409, issued Dec. 22, 1981 to N. L. Greenig et al. U.S. Pat. Nos. 4,274,703, issued June 23, 1981 to T. Fisli discloses an optical system which provides a flat scanning line, and 4,245,228, issued July 13, 1981 to G. F. Cook, discloses a laser plotter construction wherein a modulated light beam is deflected in order to scan a light sensitive medium transversely while sweeping a high resolution raster perpendicular to the scan direction in order to form points of intersection where data are to be located.